1. Field of the Invention
The present invention relates to a process for producing a buried stripe semiconductor laser, in which:
a heterostructure is formed by a first epitaxy during which successive deposition takes place on a substrate of a first confinement layer having a first doping type, an active layer and a protection layer,
the protection layer and the active layer are etched up to the first confinement layer, so as to form a stripe from the active layer and
the stripe is buried by a second epitaxy known as "repeat epitaxy" in a semiconductor layer having a second doping type opposite to the first and forming a second confinement layer.
The present invention also relates to a laser obtained by the process according to the invention.
The invention more particularly applies to the field of optical telecommunications and in particular to the production of double heterostructure semiconductor lasers for links by monomodal optical fibres.
2. Brief Description of Prior Art
Various buried stripe laser structures are already known and reference is e.g. made to:
embedded buried heterostructure (EMBH) with blocking junctions,
the flat surface buried heterostructure (FBH),
the semi-insulating planar buried heterostructure (SI-PBH) with semi-insulating layers and
the buried ridge stripe (BRS) structure with homojunctions or heterojunctions.
The growth processes and the static and dynamic characteristics of said laser structures vary very significantly.
However, the BRS structure would appear to be very promising to the extent that it only requires two successive epitaxy cycles and is suitable for all epitaxy methods such as metalorganic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE).
Various known BRS structures are described in documents (1) to (4), which are given at the end of the description and to which reference should be made.
It is also known that Thomson CSF and C.N.E.T. (Centre National d'Etudes des Telecommunications) are at present studying BRS structure lasers emitting at 1.3 and 1.5 micrometer. An example of such BRS structure lasers is diagrammatically shown in FIG. 1.
The known structure shown in FIG. 1 is obtained with the aid of two epitaxy cycles.
In a first epitaxy cycle on a n.sup.+ doped InP substrate 2 is formed a first n doped InP confinement layer 4 and then an undoped InGaAsP active layer 6, followed by a p doped InP protection layer having a limited thickness. This is followed by the etching of a stripe over a width of e.g. 2 micrometers up to the first confinement layer 4 and using a bromine-based chemical solution.
A second epitaxy cycle makes it possible to grow again on the stripe a second p doped InP confinement layer 8, followed by a p.sup.+ doped InGaAs contact layer 10.
The localization of the electric current on the mesa resulting from the etching is then obtained by the implantation of protons in the contact layer 10 and in the second confinement layer 8, which produces high resistivity regions 12 and 14 on either side of the active stripe formed. Finally, on the contact layer 10 is deposited a platinum layer 16 and then a gold layer 18.
The production process of the BRS structure described with reference to FIG. 1, although interesting in certain respects (particularly for the ease with which it can be carried out and its compatibility with all growth processes, particularly in the vapour phase) still suffers from disadvantages.
Thus, as the etching of the stripe takes place chemically in solution, problems inherent in this procedure arise during the production of the structure, particularly on large surface epitaxied semiconductor plates.
A first problem is the lateral etching of the stripe or underetching, which is characteristic of all isotropic etching operations. In view of the limited width of the mask used for the etching (2 micrometer), this phenomenon leads to a limitation of the etching depth in the material to a few hundred nanometers so as not to excessively reduce the final width of the stripe (which is typically 1.4 micrometer).
A second problem is the lack of uniformity of the etching, particularly over large surfaces where, as has been experimentally shown, chemical etching in solution initially starts on the edges of the submerged sample and spreads towards the centre thereof, thus bringing about a significant stripe depth and width variation between the sample edge and centre.
A third problem results from the fact that chemical etching in solution is known to be highly dependent on several external parameters (particularly the temperature, agitation and brightness), so that the structure obtained is difficult to reproduce.
The present invention aims at obviating the above disadvantages.